As a device which detects sleepiness of a human being, there have been proposed a variety of devices judging if a person feels sleepiness by capturing an image of a face or an eyelid of the person and catching a motion of the eyelid and/or other variations in the facial expression in the captured image. For instance, patent document 1 proposes a structure, in which the length of a time in which an eyelid is continuously opened (hereafter, referred to as “eye opened time”) is measured in captured images of an eye and its circumferential area of a subject being tested; the dispersion in the eye opened time (the standard deviation) is computed; and when the standard deviation of the eye opened time falls below a predetermined threshold value together with an increase in sleepiness, “a nap occurred” is judged. Further, in patent document 2, there has been proposed a structure in which occurrences of four sorts of facial expressions, including mouth motions in sighing, yawning, opening eyes widely and frowning, are measured from images of a subject's face, and an awakening degree, defined corresponding to an occurrence frequency of a combination of two of different sorts of the facial expressions, is outputted as an estimated result.